1. Field of the Invention
This invention relates to a collapsible, flexible sheet dam laid in a river thereacross inflated and deflated through the supply and discharge of air, and more particularly to a collapsible, flexible sheet dam laid in a place having a certain water level at a downstream side.
2. Related Art Statement
Generally, flexible sheet dams known as a collapsible rubber dam are simple to work and cheap in manufacturing cost, so that they are widely used as an intake dam for irrigation water, a barrier for damming seawater near a mouth of a river or the like.
In such a flexible sheet dam, a continuous flexible sheet (made from rubber or flexible resin) is, for example, folded into two parts and both longitudinal free edges thereof are aligned to each other, and then the aligned edge portion is directed toward an upstream side of a river and straightly secured to a riverbed face through a keep member extending in the widthwise direction of the river to form a closed bag-shaped body. Alternatively, the flexible sheet dam of the above construction is further secured to the riverbed face at a position separated downstream from the secured position of the aligned edge portion at an adequate interval through a keep member arranged inside the bag-shaped body and straightly extending in parallel to the secured line of the edge portion.
The latter flexible sheet dam secured at two positions of upstream and downstream sides (hereinafter referred to as a double securing-type dam) is particularly used in a place having a certain water level at the downstream side. In the double securing-type dam, when deflation is carried out by discharging air filled in the interior of the dam, there are problems to be discussed herein.
FIGS. 1a to 1c are sectional side elevations illustrating the deflection process of a double securing-type flexible sheet dam 02 laid on a concrete foundation 01 of a riverbed, respectively. As shown in FIG. 1a, the flexible sheet dam 02 is constructed by securing to the concrete foundation 01 at upstream securing line A and downstream securing line B. Further, spacer pipes 03 for the supply and discharge of air are fixed to a portion of the flexible sheet extending between the securing lines A and B (or a portion of the concrete foundation 01 inside the flexible sheet dam 02) at a certain interval, and plural spacers S are fixed to an inner surface portion of the folded flexible sheet facing the upstream side of the river at given intervals in the longitudinal direction of the flexible sheet dam 02.
When the air is discharged from the interior of the flowable sheet dam of the above construction, the deflation state of such a dam is different from that of a water-filling type flexible dam as mentioned below. At an initial deflation stage as shown in FIG. 1b, portions of the flexible sheet located above the securing lines A and B are gently deformed inward curvedly by the air discharge and the water pressure of the river to form curved portions 05 and 06, respectively. As the discharge of the air further proceeds, the curved portions 05 and 06 formed in the flexible sheet dam 02 are press-contacted with each other, and finally made into a joint palm state, whereby the damming function is lost to make ready for deflation.
Even when the flexible sheet dam 02 is in the deflatable state by the discharge of air, as shown in FIG. 1c, air spaces are formed in the interior of the dam at a joint point side D defined by joining end portions of two sheets to each other, a top site E of the dam after the formation of joint palm state, and gaps between the spacers S fixed to the inner face of the dam in addition to a lower space C defined by the sheet and spacer pipes 03, so that the filled air still remains in these air spaces in the longitudinal direction of the dam 02. As a result, buoyancy based on the remaining air in the air spaces exceeds the weight in water of the flexible sheet dam 02, so that it is difficult to forcedly deflate the dam 02. Therefore, the dam 02 is floating in water, which undesirably obstructs vessels such as fishing boat, working boat and the like using the river.